According to the American Heart Association, the recommended amount of minutes of exercise per day is thirty minutes per day, five days a week. The general recommendation is of at least 150 minutes in a week of moderate exercise, 75 minutes in a week of vigorous exercise or a combination of both types that matches this times.
So, the sentence is correct by adding "30" in the blank space: "You should strive for at least 30 minutes of exercise per day."